This invention relates to continuously variable transmissions.
Gear reductions are used to more efficiently transmit power from a power source, such as an internal combustion engine or a waterwheel, to a driven object, such as a set of automobile tires or a piece of machinery. Using fixed sized gears presents problems with shifting between gears and provides a very few gear ratios. For example, currently car transmissions have from three to six gears.
Continuously Variable Transmissions (CVT""s) can provide a greater number of potential gear ratios, and in theory can continuously vary the gear ratios within the operating range of the particular CVT design that is used. But CVT""s require specially curved surfaces, such as spherical surfaces or torroidal surfaces. Those specially curved surfaces are not only expensive to make, but result in concentrated forces that often lead to premature wear. Further, CVT designs use asymmetrically arranged parts that result in uneven loads on the parts of the transmissions which in turn cause premature wear. There is thus a need for an improved CVT using commonly available gear shapes and symmetric loading of parts in the transmission.
A continuously variable transmission (CVT) is provided that produces a variable drive ratio between an input and an output. The CVT has a main shaft rotating about a longitudinal axis. An input disk and an output disk are fixed to the shaft. Each disk has a driving surface inclined at substantially the same angle with respect to a plane orthogonal to the rotational axis. Each driving surface faces the driving surface on the other disk. An idler disk has a central hole through which the shaft extends, with the idler disk being interposed between the two end disks. Thus, the idler disk is mounted so as to rotate relative to the shaft, and the hole and shaft are configured to allow this rotation and to allow the disk to translate along a portion of an axial length of the shaft. The idler disk has two opposed driving surfaces inclined toward each other at substantially the same inclination angle as the driving surfaces on the end disks. The existence and movement of the idler are believed unusual in CVT""s.
A plurality of input rollers are provided. Each input roller has a conical driving surface, and each roller rotates about an input roller axis that extends radially outward from the rotational axis of the shaft in a first plane orthogonal to the rotational axis of the shaft. The conical driving surfaces drivingly engaging driving surfaces on the input disk and on the idler disk. The disks and rollers are arranged so that moving the rollers along the input roller axes varies the drive ratio.
A plurality of output rollers are also provided. Each output roller has a conical driving surface, and each output roller rotates about an output roller axis that extends radially outward from the rotational axis of the shaft in a second plane orthogonal to the rotational axis of the shaft. The conical driving surfaces drivingly engage driving surfaces on the output disk and on the idler disk. The disks and output rollers are arranged so that moving the output rollers along the output roller axes varies the drive ratio.
The input rollers and output rollers are symmetrically arranged about the main shaft rotational axis to provide a balanced load on the CVT preferably, there are three input rollers and three output rollers, but the number can vary according to the needs of the CVT. Each output roller is preferably mounted to an output shaft extending along the output roller axis. An output gear is mounted on the output roller shaft to rotate with the output roller. An output shaft is placed coaxially with the main shaft and is connected to an output ring gear engaging the output gears as one way to transfer power from the output gears of the CVT to an output shaft. The coaxial output shaft offers advantages in many applications.
The input roller is preferably mounted to an input shaft extending along the input roller axis, with an input gear mounted thereon to rotate with the input roller. An input shaft is placed coaxially with the main shaft and is connected to an input ring gear to engage the input gears as one way to transfer power from a power source to the CVT. The coaxial input shaft offers advantages in many applications.
A hydraulic piston can be connected to each output shaft to move the output shaft along the output roller axis. Likewise, a hydraulic piston can be connected to each input shaft to move the input shaft along the input roller axis. Movement of the input and output rollers toward and away from the main shaft alters the drive ratio. Hydraulic actuation helps ensure the rollers move at the same time and at the same rate of movement.
A useful angle of inclination is between about 7-30 degrees, with 10-14 degrees being a preferred range, and an angle of about 12 degrees being believed most useful for most applications. Angles of about 30 degrees or less are believed usable but the success will vary with the application.
Advantageously, but optionally, the main shaft has distal ends that are held in recesses adapted to allow the main shaft to rotate about and translate along the rotational axis of the main shaft. That axial movement, along with the axial movement of the idler disk, further accommodates radial movement of the input and output rollers as the drive ratio is varied.
In a further embodiment, the CVT includes a main shaft rotating about a longitudinal axis of the main shaft and having an input disk and an output disk fixedly mounted thereon to rotate and translate with the main shaft. An idler disk is interposed between the input and output disks. The idler disk can rotate about the main shaft and translate along the longitudinal axis of a portion of the main shaft. The input and output disks each have an annular driving surface inclined at an angle with respect to the vertical that is less than about 30 degrees. The idler disk has two annular and opposing surfaces each forming an inclined driving surface and each facing a driving surface on one of the input and output disks.
The main shaft has opposing, first and second distal ends, with a first recess receiving the first distal end of the main shaft and allowing the first distal end to freely rotate and to translate within the first recess. A second recess receives the second distal end of the main shaft and allows the second distal end to freely rotate and to translate within the second recess. The translation of the shaft further accommodates radial movement of input and output rollers described hereinafter.
A plurality of conical input rollers are provided and have rotational axes that are symmetrically arranged around the main shaft in a first plane orthogonal to that shaft and engaging the driving surface of the input disk and one driving surface of the idler disk. A plurality of conical output rollers are also provided and have rotational axes that are symmetrically arranged around the main shaft in a second plane orthogonal to that shaft and engaging the driving surface of the output disk and one driving surface of the idler disk. Movement of the rollers along the axis of rotation of the rollers varies the drive ratio. Hydraulic means can be provided for moving the rollers along the axes about which the rollers rotate. That arrangement allows the rollers to move radially toward and away from the rotating main shaft to vary the drive ratio, while gears rotating with the rollers remain in place and rotate to transmit power through the CVT.
Alternatively, mechanical means can be provided for moving the rollers along the axes about which the rollers rotate. The mechanical means preferably involve screw threads cooperating with shafts to which the rollers are mounted.
In a still further embodiment, the CVT includes a main shaft having a longitudinal rotational axis and having three disks mounted on the main shaft. The disks include a fixed input disk with a conical, input engaging surface; a fixed output disk with a conical, output engaging surface; and an idler disk having a conical engaging surface on each of two opposing sides. Each engaging surface of the idler disk faces one of the other engaging surfaces, with the idler disk rotating about and translating along a portion of the main shaft.
Input rollers, having input rotational axes located in a first plane orthogonal to the longitudinal axis, drivingly engage the rotational surfaces on the input disk and the idler disk. The input rollers have conical surfaces and are located to achieve that engagement. Output rollers, having output rotational axes located in a second plane orthogonal to the longitudinal axis, drivingly engage the rotational surfaces on the output disk and the idler disk. The output rollers have conical surfaces and are located to achieve that engagement.
Each input roller is connected to an input roller shaft that rotates with the input roller and to which is fastened an input gear that also rotates with the input roller shaft. The input gear slides along a length of the input roller shaft. Likewise, the output roller is connected to an output roller shaft that rotates with the output roller and to which is fastened an output gear that also rotates with the output roller shaft. The output gear slides along a length of the output roller shaft. That arrangement allows the rollers to move radially toward and away from the rotating main shaft to vary the drive ratio, while the gears remain in place and rotate to transmit power through the CVT.
The power output from the CVT can be taken from any of the shafts that rotate with the output rollers. A splined or geared surface rotating with the output rollers readily provides a mechanism for transmitting power from the output rollers.
The CVT uses flat, conical surfaces which are more easily produced than the complex shapes of engaging surfaces used in prior CVT""s. The CVT uses a floating idler disk to balance the loads on the CVT, allowing the use of smaller bearings and lighter parts. The CVT can use a floating main shaft to further allow for alignment and engagement of the rotating disks and rollers. Because radial movement of the rollers varies the drive ratio, a large variety of drive ratios can be achieved merely by scaling the disks and rollers to the appropriate size. Alternatively, a plurality of CVT""s could be cascaded in order to achieve a variety of drive ratios.